Measurement of Entrained and Dissolved Gases in Process Flow Lines

ABSTRACT

A device for measurement of entrained and dissolved gas has a first module arranged in relation to a process line for providing a first signal containing information about a sensed entrained air/gas in a fluid or process mixture flowing in the process line at a process line pressure. The device features a combination of a bleed line, a second module and a third module. The bleed line is coupled to the process line for bleeding a portion of the fluid or process mixture from the process line at a bleed line pressure that is lower than the process pressure. The second module is arranged in relation to the bleed line, for providing a second signal containing information about a sensed bleed line entrained air/gas in the fluid or process mixture flowing in the bleed line. The third module responds to the first signal and the second signal, for providing a third signal containing information about a dissolved air/gas flowing in the process line based on a difference between the sensed entrained air/gas and the sensed bleed line entrained air/gas.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a divisional application of U.S. patent applicationSer. No. 11/500,073 filed on Aug. 7, 2006, which is a continuation ofU.S. patent application Ser. No. 10/762,409 filed on Jan. 21, 2004, andnow U.S. Pat. No. 7,086,278 which claimed the benefit of U.S.Provisional Patent Application No. 60/482,516 filed Jun. 24, 2003,(Attorney Docket CC-0604); U.S. Provisional Patent Application No.60/441,652 filed Jan. 22, 2003, (Attorney Docket CC-0585); U.S.Provisional Patent Application No. 60/441,395 filed Jan. 21, 2003,(Attorney Docket CC-0581); all of which are incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention generally relates to a device for measuringdissolved air in a fluid or process mixture flowing in a process line.

2. Description of Related Art

Monitoring levels of entrained and dissolved gases is desirable in manyindustrial processes. For example, entrained and dissolve gases in theapproach system of paper making machines are often problematic, leadingto a wide variety of problems, including flow line pulsations, pin-holesin the produced paper, reduced paper sheet strength, and excessivebuild-up of aerobic growths.

Entrained gases are gases that exist in a gaseous form, mixed in theprocess fluid. For many industrial applications with small, less than˜20% gas fraction by volume, the gas is typically in the form of smallbubbles contained in a liquid continuous mixture. Entrained gases existas either free bubbles moving within the stock or as bound (or residual)air that is adhered to the fiber. In either cases, entrained air cangenerally be detected by monitoring the compressibility of the mixtureand correlating the compressibility to volumetric percentage ofentrained air.

Dissolved gases are dissolved within the mixture on a molecular level.While in the solution, dissolved gases pose few operation problems.Typically dissolved gases have a negligible effect on thecompressibility of the mixture. Thus, dissolved gases are difficult todetect via compressibility measurements.

Although dissolved gases are typically not problematic while dissolved,problems arise when dissolved gases come out of a solution as a resultof either decreases in pressure or increases in temperature. One exampleof this is in pressurized head boxes on paper machines where thepressure drop associated with spraying the pulp/water mixture on to thepaper machine can cause dissolved gases to come out of the solution andform entrained air.

Thus, to accurately monitor problems associated with entrained anddissolve gases, it is desirable to be able to measure both quantities.

SUMMARY OF THE INVENTION

In its broadest sense, the present invention provides a new and uniquedevice having a first module arranged in relation to a process line forproviding a first signal containing information about a sensed entrainedair/gas in a fluid or process mixture flowing in the process line at aprocess line pressure. The device features a combination of a bleedline, a second module and a third module. The bleed line is coupled tothe process line for bleeding a portion of the fluid or process mixturefrom the process line at a bleed line pressure that is lower than theprocess pressure. The second module is arranged in relation to the bleedline, for providing a second signal containing information about asensed bleed line entrained air/gas in the fluid or process mixtureflowing in the bleed line. The third module responds to the first signaland the second signal, for providing a third signal containinginformation about a dissolved air/gas flowing in the process line basedon a difference between the sensed entrained air/gas and the sensedbleed line entrained air/gas.

In one embodiment, the first module is a primary process line entrainedair measurement module that includes an array of sensors that measuresthe speed of sound propagating through the fluid or process mixtureflowing within the process line and determines the entrained air basedon a measurement using the speed of sound. The second module is a bleedline entrained air measurement module that also includes an array ofsensors that measures the speed of sound propagating through the fluidor process mixture flowing within the bleed line and determines thebleed line entrained air based on a measurement using the speed ofsound. The third module is a dissolved air determination processormodule that processes the first and second signals and provides thethird signal containing information about a dissolved air/gas flowing inthe process line.

The device also includes a bleed line control module for controlling thebleeding off of the portion of the fluid from the process line via ableed valve and the reinjection of said portion back to the process linevia a boost pump, and also includes a controller module for controllingand coordinating the operation of the first, second and third modules,as well as a bleed line control that communicates with the bleed valveand the boost pump.

In operation, the device according to the present invention measuresdissolved gases at an operating pressure by measuring entrained gasespresent in a process line once the fluid or process mixture is expandedto ambient (or other known and relevant) pressure. This measurement isperformed using a small amount of process mixture bled-off eithercontinuously or periodically, from the process. The bled-off processfluid can be recirculated or, via a boost pump, re-pressurized andreinjected. The bleed line and flow rates may be sized to minimize theamount of stock bleed off while maintained sufficiently high flow ratesto maintain sufficiently homogenous flow within the bled-off liquid testsection (i.e. minimize slip) such that the measured gas volume fractionwithin the bleed line is indeed representative of the amount of gasdissolved in the process fluid. Maintaining sufficiently high velocitiesavoids problems associated with stratification of the mixture and theproblems associated with either the liquid of gas phases “holding up” inthe process pipe. For most mixtures of liquids and gases at or nearambient pressures, flow velocities of several feet per second throughthe line are sufficient.

The process of the throttling of the process fluid to the reducedpressure provides sufficient noise to perform a sonar-based speedmeasurement.

The present invention also provides a method for measuring the entrainedgas fraction at two relevant pressures, and thus provides practicalmeasurement of the amount of both entrained and dissolved gasescontained in the process fluid at the process operating conditions.

The foregoing and other objects, features and advantages of the presentinvention will become more apparent in light of the following detaileddescription of exemplary embodiments thereof.

BRIEF DESCRIPTION OF THE DRAWING

The drawing, not drawn to scale, includes the following Figures:

FIG. 1 is a block diagram of a device for measurement of entrained anddissolved gases that is the subject matter of the present invention.

FIG. 2 is a schematic of the device shown in FIG. 1.

FIG. 3 is an overview of the system according to the present invention.

FIG. 4 is a graph of a gas volume fraction (GVF) between 0.00001 and 0.1versus a mixture sound speed in meters per see (m/s).

FIG. 5 is a graph of a gas volume fraction (GVF) between 0.0 and 0.1versus a mixture sound speed in meters per see.

FIG. 6 is a k-ω plot constructed according to the present invention,showing acoustic ridges, wherein the fluid flowing in the pipe is waterwith entrained air.

FIG. 7 is a block diagram of an apparatus for measuring entrained air ina fluid flowing within a pipe, such as a bleed line and primary processline, in accordance with the present invention.

FIG. 8 is a block diagram of another embodiment of an apparatus formeasuring entrained air in a fluid flowing within a pipe, such as ableed line and primary process line, in accordance with the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

FIGS. 1 and 2 show a schematic and block diagram of a device generallyindicated as 10 for measurement of entrained and dissolved gases in afluid or process mixture generally indicated as 11 flowing in a primaryprocess line 12 having a given process pressure.

In FIG. 1, the device 10 includes a first, or primary process line,entrained air measurement module 14, a bleed line 16 (see also FIG. 2),a second, or bleed line, entrained air measurement module 18 and adissolved air/gas determination processor module 20. The primary processline entrained air measurement module 14 is arranged in relation to theprimary process line 12, for sensing entrained air in the fluid orprocess mixture 11 and providing a first entrained air measurementmodule signal via line 14 a containing information about sensed primaryprocess line entrained air. As best shown in FIG. 2, the bleed line 16is coupled to the primary process line 12 for bleeding fluid or processmixture from the primary process line 12 at a bleed line pressure thatis lower than the process pressure, for example, at ambient pressure. Asbest shown in FIG. 1, the second entrained air measurement module 18 isarranged in relation to the bleed line 16, for sensing entrained air inthe fluid or process mixture in the bleed line, and providing a secondentrained air measurement module signal via line 18 a containinginformation about sensed bleed line entrained air. In FIG. 1, thedissolved air/gas determination processor module 20 responds to thefirst entrained air measurement module signal along line 14 a and thesecond entrained air measurement module signal along line 18 a, eachsignal being received via a controller module 22 as shown and discussedbelow; determines dissolved air/gas in the fluid or process mixtureflowing in the primary process line based on a difference between thesensed primary process line entrained air and the sensed bleed lineentrained air; and provides a dissolved air/gas determination processormodule signal containing information about the same.

The controller module 22 controls and coordinates the operation of themodules 14, 18, 20 and 32. As shown, the signals along lines 14 a, 18 aare provided directly to the controller module 22, although the scope ofthe invention is intended to include embodiments in which the signalsalong lines 14 a, 18 a are provided directly to the dissolved air/gasdetermination processor module 20. The bleed line control module 32controls the bleed valve 28 and boost pump 30 (see FIG. 2) in responseto a signal from the controller module 22.

The primary process line entrained air measurement module 14 includes anarray of sensors 24 shown in FIG. 2 that measures the speed of soundpropagating through the fluid or process mixture flowing within theprocess line 12 and determines the entrained air based on a measurementusing the speed of sound, as will be described in greater detailhereinafter.

Similarly, the bleed line entrained air measurement module 18 includes acorresponding array of sensors 26 that measures the speed of soundpropagating through the fluid or process mixture 11 flowing within thebleed line 16 and determines the bleed line entrained air based on ameasurement using the speed of sound.

The bleed line 16 has a bleed valve 28 for bleeding the fluid or processmixture 11 into the bleed line 16. The bleed line 16 is re-coupled tothe primary process line 12 via a boost pump 30 to recirculate the fluidor process mixture bled therefrom. The scope of the invention is notintended to be limited to the type or kind of bleed valve or boost pumpused. The invention is shown and described in relation to a closed loopsystem; however, the scope of the invention is intended to include anopen loop system in which the media from the bleed line is not returnedto the process line.

The device 10 also includes a bleed line control module 32 forcontrolling the bleeding off of the portion of the fluid or processmixture from the process line via the bleed valve 28 and the reinjectionof the same back to the process line 12 via the boost pump 30.

The modules 14, 18, 20, 22, 32 may be implemented using hardware,software, or a combination thereof. The scope of the invention is notintended to be limited to any particular implementation thereof. Forexample, a typical software implementation may include using amicroprocessor architecture having a microprocessor, a random accessmemory (RAM), a read only memory (ROM), input/output devices and acontrol, address and databus for connecting the same.

Although the invention is described in relation to measuring or sensingentrained air in a fluid or process mixture using an array of sensors,the scope of the invention is intended to include other ways ofmeasuring or sensing entrained air either known now or developed in thefuture. Moreover, although the invention is described in relation tousing an array of sensors to determine the speed of sound, the scope ofthe invention is intended to include other ways of measuring the speedof sound either known now or developed in the future.

Entrained Gas Measurement

The present invention uses the speed at which sound propagates within aconduit to measure entrained air in slurries. This approach may be usedwith any technique that measures the sound speed of a fluid or processmixture. However, it is particularly synergistic with sonar basedvolumetric flow meters such as described in aforementioned U.S. patentapplication, Ser. No. 10/007,736 (CiDRA's Docket No. CC-0122A), now U.S.Pat. No. 6,889,562, in that the sound speed measurement, and thus gasvolume fraction measurement, can be accomplished using the same hardwareas that required for the volumetric flow measurement. It should benoted, however, that the gas volume fraction (GVF) measurement could beperformed independently of a volumetric flow measurement, and would haveutility as an important process measurement in isolation or inconjunction with other process measurements.

Firstly, the sound speed may be measured as described in aforementionedU.S. patent applications, Ser. No. 09/344,094 (CiDRA's Docket No.CC-0066A), now U.S. Pat. No. 6,354,147, Ser. No. 10/007,749 (CiDRA'sDocket No. CC-0066B), now U.S. Pat. No. 6,732,575, U.S. patentapplication, Ser. No. 10/349,716 filed Jan. 23, 2003 (Cidra's Docket No.CC-0579) and/or U.S. patent application, Ser. No. 10/376,427 filed Feb.26, 2003 (Cidra's Docket No. CC-0596), now U.S. Pat. No. 7,032,432 allincorporated herein by reference, using an array of unsteady pressuretransducers. For a two component mixture, utilizing relations describedin U.S. patent applications, Ser. No. 09/344,094 (CiDRA's Docket No.CC-0066A), now U.S. Pat. No. 6,354,147, and/or Ser. No. 10/007,749(CiDRA's Docket No. CC-0066B), now U.S. Pat. No. 6,732,575, knowledge ofthe density and sound speed of the two components and the complianceproperties of the conduit or pipe, the measured sound speed can be usedto determine the volumetric phase fraction of the two components.

The sound speed of a mixture can be related to volumetric phase fraction(φ_(i)) of the components and the sound speed (a) and densities (ρ) ofthe component through the Wood equation, where

$\frac{1}{\rho_{mix}a_{{mix}\mspace{11mu} \infty}^{2}} = {{\sum\limits_{i = 1}^{N}{\frac{\varphi_{i}}{\rho_{i}a_{i}^{2}}\mspace{14mu} {where}\mspace{14mu} \rho_{mix}}} = {\sum\limits_{i = 1}^{N}{\rho_{i}\varphi_{i}}}}$

One dimensional compression waves propagating within a fluid containedwithin a conduit exert an unsteady internal pressure loading on theconduit. The degree to which the conduit displaces as a result of theunsteady pressure loading influences the speed of propagation of thecompression wave. The relationship among the infinite domain speed ofsound and density of a fluid; the elastic modulus (E), thickness (t),and radius (R) of a vacuum-backed cylindrical conduit; and the effectivepropagation velocity (a_(eff)) for one dimensional compression is givenby the following expression:

${ae}_{ff} = \frac{1}{\sqrt{\frac{1}{a_{{mix}\mspace{11mu} \infty}^{2}} + {\rho_{mix}\frac{2R}{Et}}}}$

Note: “vacuum backed” as used herein refers to a situation in which thefluid surrounding the conduit externally has negligible acousticimpedance compared to that of the fluid internal to the pipe. Forexample, meter containing a typical water and pulp slurry immersed inair at standard atmospheric conditions satisfies this condition and canbe considered “vacuum-backed”.

For paper and pulp slurries, the conditions are such that for slurrieswith non-negligible amounts of entrained gas, say <0.01%, the complianceof standard industrial piping (Schedule 10 or 40 steel pipe) istypically negligible compared to that of the entrained air.

FIGS. 4 and 5 show the relationship between sound speed and entrainedair for slurries with pulp contents representative of the range used inthe paper and pulp industry. Referring to FIG. 4, two slurryconsistencies are shown; representing the lower limit, a pure watermixture is considered, and representing the higher end of consistencies,a 5% pulp/95% water slurry is considered. Since the effect of entrainedair on the sound speed of the mixture is highly sensitive to thecompressibility of the entrained air, the effect of the entrained air isexamined at two pressures, one at ambient representing the lower limitof pressure, and one at four atmospheres representing a typical linepressure in a paper process. As shown, the consistency of the liquidslurry, i.e., the pulp content, has little effect on the relationshipbetween entrained air volume fraction and mixture sound speed. Thisindicates that an entrained air measurement could be accuratelyperformed, within 0.01% or so, with little or no knowledge of theconsistency of the slurry. The chart does show a strong dependence online pressure. Physically, this effect is linked to the compressibilityof the air, and thus, this indicates that reasonable estimates of linepressure and temperature would be required to accurately interpretmixture sound speed in terms of entrained air gas volume fraction.

FIG. 4 also shows that for the region of interest, from roughly 1%entrained air to roughly 5% entrained air, mixture sound speeds(a_(mix)) are quite low compared to the liquid-only sound speeds. In theexample shown above, the sound speed of the pure water and the 5% pulpslurry were calculated, based on reasonable estimates of the constituentdensities and compressibilities, to be 1524 m/s and 1541 m/s,respectively. The sound speed of these mixtures with 1% to 5% entrainedair at typical operating pressure (1 atm to 4 atms) are on the order of100 m/sec. The implication of these low sound speeds is that the mixturesound speed could be accurately determined with an array of sensors,i.e. using the methodology described in aforementioned U.S. patentapplications, Ser. No. 09/344,094 (CiDRA's Docket No. CC-0066A), nowU.S. Pat. No. 6,354,147, and/or Ser. No. 10/007,749 (CiDRA's Docket No.CC-0066B), now U.S. Pat. No. 6,732,575, with an aperture that issimilar, or identical, to an array of sensors that would be suitable todetermine the convection velocity, using the methodology described inaforementioned U.S. patent application, Ser. No. 10/007,736 (CiDRA'sDocket No. CC-0122A), now U.S. Pat. No. 6,889,562, which is incorporatedherein by reference.

A flow chart of the proposed measurement is shown in FIG. 3, where theinputs are the mixture of SOS, P and T are pressure and temperature,respectively, and GVT air (gas volumetric flow of air) is provided fromthe box “Entrained Air Volume Fraction” as an output and to the box“correct for void fraction of air” and Q mixture (volumetric flow of themixture) is provided from the box “Total Mixture Flow Rate” as an outputand to the box “correct for void fraction of air”.

Other information relating to the gas volume fraction in a fluid and thespeed of sound (or sonic velocity) in the fluid, is described in “FluidMechanics and Measurements in two-phase flow Systems”, Institution ofmechanical engineers, proceedings 1969-1970 Vol. 184 part 3C, Sep. 24-251969, Birdcage Walk, Westminster, London S.W. 1, England.

Based on the above discussion, one may use a short length scale apertureto measure the sound speed.

The characteristic acoustic length scale is: λ=c/f; where c is the speedof sound in a mixture, f is frequency and λ is wavelength.

If Aperture=L and if L/λ is approx. constant.

Then Lwater/λwater=Lwater*f/C_(water)≈L_(GVF)*f/c_(GVF)

Therefore: L_(GVF)=Lwater (C_(GVF)/C_(water)); where GVF is gas volumefraction.

Thus for SOS of water (Cwater=5,000 ft/sec), and SOS of the Gas volumefraction (C GVF=500 ft/sec) and a length aperture of L water=5 ft (whichwe have shown is sufficient to accurately measure the SOS of water), thelength aperture for a gas volume fraction L_(GVF) would be about 0.5feet.

Note that this entrained air or gas volume fraction measurement GVFairmay be used with any flow meter or consistency meter to correct forerrors introduced into a measurement by entrained air. In particular, anelectromagnetic flow meter will show an error when entrained air existsin the mixture. The present invention may be used to correct for thiserror. In addition, a consistency meter will show an error whenentrained air exists in the mixture. The present invention may be usedto correct for this error.

The scope of the invention is also intended to include using othermodels and corrections for determining entrained air in a fluid that maybe used to compensate for gas volume fraction.

As shown in FIG. 3, the sonar meter measures the speed at which acousticwave propagating in the process piping to determine the amount ofentrained air in the process line. The acoustic wave can be generated bya pump or other device disposed in the piping system, or generatedsimply by the mixture/fluid flowing through the pipe, all of whichprovide a passive acoustic source. Alternatively, the sonar flow meterincludes an active acoustic source that injects an acoustic wave intothe flow such as by compressing, vibrating and/or tapping the pipe, toname a few examples.

e flow such as by compressing, vibrating and/or tapping the pipe, toname a few examples.

The connection between speed of sound of a two-phase mixture and phasefraction is well established for mixtures in which the wavelength of thesound is significantly larger than any inhomogenieities, i.e. bubbles,in the flow.

The mixing rule essentially states that the compressibility of a mixture(1/(ρ a²)) is the volumetrically-weighted average of thecompressibilities of the components. For gas/liquid mixtures at pressureand temperatures typical of paper and pulp industry, the compressibilityof gas phase is orders of magnitudes greater than that of the liquid.Thus, the compressibility of the gas phase and the density of the liquidphase primarily determine mixture sound speed, and as such, it isnecessary to have a good estimate of process pressure to interpretmixture sound speed in terms of volumetric fraction of entrained air Theeffect of process pressure on the relationship between sound speed andentrained air volume fraction is shown in FIG. 4.

Conversely, however, detailed knowledge of the liquid/slurry is notrequired for entrained air measurement. Variations in liquid density andcompressibility with changes in consistency have a negligible effect onmixture sound speed compared to the presence of entrained air. FIG. 5shows the mixture sound speed as a function of entrained air volumefraction for two slurries, one with 0% wood fiber and the other with 5%wood fiber by volume. As shown, the relationship between mixture soundspeed and gas volume fraction is essentially indistinguishable for thetwo slurries Furthermore, mixture sound speed is shown to an excellentindicator of gas volume fraction, especially for the trace to moderateamounts of entrained air, from 0 to 5% by volume, typically encounteredin the paper and pulp industry.

Speed of Sound Measurement

As mentioned earlier, the relationship between mixture sound speed andentrained air in bubbly liquids is well established. However, as will bedeveloped below, in bubbly flows, these relations are only applicablefor the propagation of relatively low frequency, long wavelength sound.While this restriction does not present any significant obstacles forthe sonar meter, it does present significant challenges to ultrasonicsound speed measurement devices.

Ultrasonic meters typically operate in 100 Khz to several Mhz frequencyrange. For these meters, entrained air bubbles have length scales on thesame order as the acoustic waves generated by the ultrasonic meters.They posed several problems. Firstly, the bubbles scatter the ultrasonicwaves, impairing the ability of the ultrasonic meter to perform a soundspeed measurement. Also, ultrasonic meters rely on information derivedfrom only a small fraction of the cross sectional area of the pipe to berepresentative of the entire cross section, an assumption that breaksdown for flows with inhogenieties on the same length scale as theultrasonic wavelength.

Sonar flow meters use an approach developed and commercializedspecifically for multiphase flow measurement in the oil and gasindustry. Sonar meters measure the propagation velocity of operationallygenerated sound in the ˜100 to 1000 Hz frequency range. In thisfrequency range, sound propagates as a one-dimensional wave using theprocess pipe as a waveguide. The wavelength of sound in this frequencyrange (>1 m) is typically several orders of magnitude larger than thelength scale of the any bubbles. The long wavelength acoustics propagatethrough the bubbles unimpeded, providing a robust and representativemeasure of the volumetrically averaged properties of the flow.

For the sound speed measurement, the sonar flow meter utilizes similarprocessing algorithms as those employed for the volumetric flowmeasurement. As with convective disturbances, the temporal and spatialfrequency content of sound propagating within the process piping isrelated through a dispersion relationship.

k=ω/a_(mix)

As before, k is the wave number, defined as k=2π/λ, ω is the temporalfrequency in rad/sec, and a_(mix) is the speed at which sound propagateswithin the process piping. Unlike disturbances which convect with theflow, however, sound generally propagates in both directions, with andagainst the mean flow. For these cases, the acoustic power is locatedalong two acoustic ridges, one for the sound traveling with the flow ata speed of a_(mix)+V_(mix) and one for the sound traveling against theflow at a speed of a_(mix)-V_(mix).

FIG. 6 shows a k-ω plot generated for acoustic sound field recorded fromwater flowing at a rate of 240 gpm containing ˜2% entrained air byvolume in a 3 inch, schedule 10, stainless steel pipe. The k-ω plot wasconstructed using data from an array of strain-based sensors attached tothe outside of the pipe. Two acoustic ridges are clearly evident. Basedon the slopes of the acoustic ridges, the sound speed for this mixturewas 330 ft/sec (100 m/s), consistent with that predicted by the Woodequation. Note that adding 2% air by volume reduces the sound speed ofthe bubbly mixture to less than 10% of the sound speed of single phasewater. FIG. 7 illustrates a schematic drawing of one embodiment of thepresent invention. The apparatus 210 includes a sensing device 216comprising an array of pressure sensors (or transducers) 218-221 spacedaxially along the outer surface 222 of a pipe 214, having a process flowpropagating therein, similar to that described hereinbefore. Thepressure sensors measure the unsteady pressures produced by acousticaldisturbances within the pipe, which are indicative of the SOSpropagating through the mixture 212. The output signals (P₁-P_(N)) ofthe pressure sensors 218-221 are provided to the processor 224, whichprocesses the pressure measurement data and determines the speed ofsound, gas volume fraction (GVF) and other parameters of the flow asdescribed hereinbefore.

In an embodiment of the present invention shown in FIG. 7, the apparatus210, similar to the arrays 24,26 of FIG. 2, has at least two pressuresensors 218-221 disposed axially along the pipe 214 for measuring theunsteady pressure P₁-P_(N) of the mixture 212 flowing therethrough. Thespeed of sound propagating through the flow 212 is derived byinterpreting the unsteady pressure field within the process piping 214(e.g., the bleed line 16 and primary process line 12) using multipletransducers displaced axially over ˜2 diameters in length. The flowmeasurements can be performed using ported pressure transducers orclamp-on, strain-based sensors.

The apparatus 210 has the ability to measure the gas volume fraction andother parameters by determining the speed of sound of acousticaldisturbances or sound waves propagating through the flow 212 using thearray of pressure sensors 218-221.

Generally, the apparatus 210 measures unsteady pressures created byacoustical disturbances propagating through the flow 212 to determinethe speed of sound (SOS) propagating through the flow, Knowing ormeasuring the pressure and/or temperature of the flow by a pressuresensor 23 and a temperature sensor 25, respectively, and the speed ofsound of the acoustical disturbances, the processing unit 224 candetermine the gas volume fraction of the mixture, similar to that shownin U.S. patent application Ser. No. 10/349,716 (Cidra Docket No.CC-0579), filed Jan. 21, 2003, now U.S. Publication 2003/0154036, U.S.patent application Ser. No. 10/376,427 (Cidra Docket No. CC-0596), filedFeb. 26, 2003, now U.S. Pat. No. 7,032,432, and U.S. Provisional PatentApplication Ser. No. 60/528,802 (Cidra Docket No. CC-0685), filed Dec.11, 2003 which are all incorporated herein by reference.

The apparatus 210 in FIG. 7 also contemplates providing one or moreacoustic sources 227 to enable the measurement of the speed of soundpropagating through the flow for instances of acoustically quiet flow.The acoustic sources may be disposed at the input end or output end ofthe array of sensors 218-221, or at both ends as shown. One shouldappreciate that in most instances the acoustics sources are notnecessary and the apparatus passively detects the acoustic ridgeprovided in the flow 212. The passive noise includes noise generated bypumps, valves, motors, and the turbulent mixture itself.

The apparatus 210 of the present invention may be configured andprogrammed to measure and process the detected unsteady pressuresP₁(t)-P_(N)(t) created by acoustic waves propagating through the mixtureto determine the SOS through the flow 212 in the pipe 214. One suchapparatus 310 is shown in FIG. 8 that measures the speed of sound (SOS)of one-dimensional sound waves propagating through the mixture todetermine the gas volume fraction of the mixture. It is known that soundpropagates through various mediums at various speeds in such fields asSONAR and RADAR fields. The speed of sound propagating through the pipeand mixture 212 may be determined using a number of known techniques,such as those set forth in U.S. patent application Ser. No. 09/344,094,entitled “Fluid Parameter Measurement in Pipes Using AcousticPressures”, filed Jun. 25, 1999, now U.S. Pat. No. 6,354,147; U.S.patent application Ser. No. 09/729,994, filed Dec. 4, 2002, now U.S.Pat. No. 6,609,069; U.S. patent application Ser. No. 09/997,221, filedNov. 28, 2001, now U.S. Pat. No. 6,587,798; and U.S. patent applicationSer. No. 10/007,749, entitled “Fluid Parameter Measurement in PipesUsing Acoustic Pressures”, filed Nov. 7, 2001, now U.S. Pat. No.6,732,575, each of which are incorporated herein by reference.

In accordance with one embodiment of the present invention, the speed ofsound propagating through the mixture 212 is measured by passivelylistening to the flow with an array of unsteady pressure sensors todetermine the speed at which one-dimensional compression waves propagatethrough the mixture 212 contained within the pipe 214.

As shown in FIG. 8, an apparatus 310 embodying the present invention hasan array of at least two acoustic pressure sensors 115,116, located atthree locations x₁,x₂ axially along the pipe 214. One will appreciatethat the sensor array may include more than two pressure sensors asdepicted by pressure sensors 117,118 at location X₃,X_(N). The pressuregenerated by the acoustic waves may be measured through pressuresensors—215-218. The pressure sensors 215-218 provide pressuretime-varying signals P₁(t),P₂(t),P₃(t),P_(N)(t) on lines 120,121,122,123to a signal processing unit 130 to known Fast Fourier Transform (FFT)logics 126,127,128,129, respectively. The FFT logics 126-129 calculatethe Fourier transform of the time-based input signals P₁(t)-P_(N)(t) andprovide complex frequency domain (or frequency based) signalsP₁(ω),P₂(ω),P₃(ω)),P_(N)(ω) on lines 132,133,134,135 indicative of thefrequency content of the input signals. Instead of FFT's, any othertechnique for obtaining the frequency domain characteristics of thesignals P₁(t)-P_(N)(t), may be used. For example, the cross-spectraldensity and the power spectral density may be used to form a frequencydomain transfer functions (or frequency response or ratios) discussedhereinafter.

The frequency signals P₁(ω)-P_(N)(ω) are fed to an array processing unit138, similar to the array processing unit 223, which provides a signalto line 140 indicative of the speed of sound of the mixture a_(mix),discussed more hereinafter. The a_(mix) signal, temperature signal (fromtemperature sensor 4), and pressure signal (from pressure sensor 3) isprovided to an entrained gas processing unit 142, similar to theprocessing unit 225. The processing unit 142 converts a_(mix) to apercent composition of a mixture and provides a gas volume fraction or %Comp signal to line 144 indicative thereof (as discussed hereinafter).

The data from the array of sensors 115-118 may be processed in anydomain, including the frequency/spatial domain, the temporal/spatialdomain, the temporal/wave-number domain or the wave-number/frequency(k-ω) domain. As such, any known array processing technique in any ofthese or other related domains may be used if desired, similar to thetechniques used in the fields of SONAR and RADAR.

One such technique of determining the speed of sound propagating throughthe flow 212 is using array processing techniques to define an acousticridge in the k-ω plane as shown in FIG. 6. The slope of the acousticridge is indicative of the speed of sound propagating through the flow212. This technique is similar to that described in U.S. Pat. No.6,587,798 filed Nov. 28, 2001, titled “Method and System for DeterminingThe Speed of Sound in a Fluid Within a Conduit”, which is incorporatedherein by reference. The speed of sound (SOS) is determined by applyingsonar arraying processing techniques to determine the speed at which theone dimensional acoustic waves propagate past the axial array ofunsteady pressure measurements distributed along the pipe 214.

The signal processor 224 performs a Fast Fourier Transform (FFT) of thetime-based pressure signals P₁(t)-P_(N)(t) to convert the pressuresignal into the frequency domain. The power of the frequency-domainpressure signals are then determined and defined in the k-ω plane byusing array processing algorithms (such as Capon and Music algorithms).The acoustic ridge in the k-ω plane, as shown in the k-ω plot of FIG. 6,is then determined. The speed of sound (SOS) is determined by measuringslope of the acoustic ridge. The gas volume fraction is then calculatedor otherwise determined, as described hereinafter.

The flow meter of the present invention uses known array processingtechniques, in particular the Minimum Variance, Distortionless Responseor other adaptive array processing techniques (MVDR, Music, or Capontechnique), to identify pressure fluctuations, which convect with thematerials flowing in a conduit and accurately ascertain the velocity,and thus the flow rate, of said material. These processing techniquesutilize the covariance between multiple sensors 218-221 at a pluralityof frequencies to identify signals that behave according to a givenassumed model; in the case of the apparatus 310, a model, whichrepresents pressure variations convecting at a constant speed across thepressure sensors comprising the sensing device 216.

Also, some or all of the functions within the processor 130 may beimplemented in software (using a microprocessor or computer) and/orfirmware, or may be implemented using analog and/or digital hardware,having sufficient memory, interfaces, and capacity to perform thefunctions described herein.

For certain types of pressure sensors, e.g., pipe strain sensors,accelerometers, velocity sensors or displacement sensors, discussedhereinafter, it may be desirable for the pipe 214 to exhibit a certainamount of pipe compliance.

The pressure sensors 218-221 described herein may be any type ofpressure sensor, capable of measuring the unsteady (or ac or dynamic )pressures within a pipe, such as piezoelectric, optical, capacitive,resistive (e.g., Wheatstone bridge), accelerometers (or geophones),velocity measuring devices, displacement measuring devices, etc. Ifoptical pressure sensors are used, the sensors 218-221 may be Bragggrating based pressure sensors, such as that described in U.S. patentapplication, Ser. No. 08/925,598, entitled “High Sensitivity Fiber OpticPressure Sensor For Use In Harsh Environments”, filed Sep. 8, 1997, nowU.S. Pat. No. 6,016,702, which are incorporated herein by reference.Alternatively, the sensors 218-221 may be electrical or optical straingages attached to or embedded in the outer or inner wall of the pipewhich measure pipe wall strain, including microphones, hydrophones, orany other sensor capable of measuring the unsteady pressures within thepipe 214. In an embodiment of the present invention that utilizes fiberoptics as the pressure sensors 218-221, they may be connectedindividually or may be multiplexed along one or more optical fibersusing wavelength division multiplexing (WDM), time division multiplexing(TDM), or any other optical multiplexing techniques.

For any of the embodiments described herein, the pressure sensors,including electrical strain gages, optical fibers and/or gratings amongothers as described herein, may be attached to the pipe by adhesive,glue, epoxy, tape or other suitable attachment means to ensure suitablecontact between the sensor and the pipe 214. The sensors mayalternatively be removable or permanently attached via known mechanicaltechniques such as mechanical fastener, spring loaded, clamped, clamshell arrangement, strapping or other equivalents. Alternatively, thestrain gages, including optical fibers and/or gratings, may be embeddedin a composite pipe. If desired, for certain applications, the gratingsmay be detached from (or strain or acoustically isolated from) the pipe212 if desired.

It is also within the scope of the present invention that any otherstrain sensing technique may be used to measure the variations in strainin the pipe, such as highly sensitive piezoelectric, electronic orelectric, strain gages attached to or embedded in the pipe 214.

In certain embodiments of the present invention a piezo-electronicpressure transducer may be used as one or more of the pressure sensors218-221 and it may measure the unsteady (or dynamic or ac) pressurevariations inside the pipe 214 by measuring the pressure levels insideof the pipe 214. In an embodiment of the present invention the sensors218-221 comprise pressure sensors manufactured by PCB Piezotronics. Inone pressure sensor there are integrated circuit piezoelectric voltagemode-type sensors that feature built-in microelectronic amplifiers, andconvert the high-impedance charge into a low-impedance voltage output.Specifically, a Model 106B manufactured by PCB Piezotronics is usedwhich is a high sensitivity, acceleration compensated integrated circuitpiezoelectric quartz pressure sensor suitable for measuring low pressureacoustic phenomena in hydraulic and pneumatic systems. It has the uniquecapability to measure small pressure changes of less tan 0.001 psi underhigh static conditions. The 106B has a 300 mV/psi sensitivity and aresolution of 91 dB (0.0001 psi).

The pressure sensors incorporate a built-in MOSFET microelectronicamplifier to convert the high-impedance charge output into alow-impedance voltage signal. The sensor is powered from aconstant-current source and can operate over long coaxial or ribboncable without signal degradation. The low-impedance voltage signal isnot affected by triboelectric cable noise or insulationresistance-degrading contaminants. Power to operate integrated circuitpiezoelectric sensors generally takes the form of a low-cost, 24 to 27VDC, 2 to 20 mA constant-current supply. A data acquisition system ofthe present invention may incorporate constant-current power fordirectly powering integrated circuit piezoelectric sensors.

Most piezoelectric pressure sensors are constructed with eithercompression mode quartz crystals preloaded in a rigid housing, orunconstrained tourmaline crystals. These designs give the sensorsmicrosecond response times and resonant frequencies in the hundreds ofkHz, with minimal overshoot or ringing. Small diaphragm diameters ensurespatial resolution of narrow shock waves.

The output characteristic of piezoelectric pressure sensor systems isthat of an AC-coupled system, where repetitive signals decay until thereis an equal area above and below the original base line. As magnitudelevels of the monitored event fluctuate, the output remains stabilizedaround the base line with the positive and negative areas of the curveremaining equal.

The pressure sensors 218-221 described herein may be any type ofpressure sensor, capable of measuring the unsteady (or ac or dynamic )pressures within a pipe, such as piezoelectric, optical, thermal,capacitive, inductive, resistive (e.g., Wheatstone bridge),accelerometers (or geophones), velocity measuring devices, displacementmeasuring devices, etc. If optical pressure sensors are used, thesensors 218-221 may be Bragg grating based pressure sensors, such asthat described in U.S. patent application, Ser. No. 08/925,598, entitled“High Sensitivity Fiber Optic Pressure Sensor For Use In HarshEnvironments”, filed Sep. 8, 1997, now U.S. Pat. No. 6,016,702.Alternatively, the sensors 218-221 may be electrical or optical straingages attached to or embedded in the outer or inner wall of the pipewhich measure pipe wall strain, including microphones, hydrophones, orany other sensor capable of measuring the unsteady pressures within thepipe 214. In an embodiment of the present invention that utilizes fiberoptics as the pressure sensors 218-221, they may be connectedindividually or may be multiplexed along one or more optical fibersusing wavelength division multiplexing (WDM), time division multiplexing(TDM), or any other optical multiplexing techniques.

A piezo-electronic pressure transducer may be used (or alternativelyeven a common strain gage may be used) as one or more of the pressuresensors 218-221, and it may measure the unsteady (or dynamic or ac)pressure variations Pin inside the pipe 214 by measuring the pressurelevels (or for the strain gage, the elastic expansion and contraction ofthe diameter of the pipe 214. In an embodiment of the present inventionthe sensors 218-221 comprise pressure sensors manufactured by PCBPiezotronics. In one pressure sensor there are integrated circuitpiezoelectric voltage mode-type sensors that feature built-inmicroelectronic amplifiers, and convert the high-impedance charge into alow-impedance voltage output. Specifically, a Model 106B manufactured byPCB Piezotronics is used which is a high sensitivity, accelerationcompensated integrated circuit piezoelectric quartz pressure sensorsuitable for measuring low pressure acoustic phenomena in hydraulic andpneumatic systems. It has the unique capability to measure smallpressure changes of less than 0.001 psi under high static conditions.The 106B has a 300 mV/psi sensitivity and a resolution of 91 dB (0.0001psi).

For any of the embodiments described herein, the pressure sensors,including electrical strain gages, optical fibers and/or gratings amongothers as described herein, may be attached to the pipe by adhesive,glue, epoxy, tape or other suitable attachment means to ensure suitablecontact between the sensor and the pipe 212. The sensors mayalternatively be removable or permanently attached via known mechanicaltechniques such as mechanical fastener, spring loaded, clamped, clamshell arrangement, strapping or other equivalents. Alternatively, thestrain gages, including optical fibers and/or gratings, may be embeddedin a composite pipe. If desired, for certain applications, the gratingsmay be detached from (or strain or acoustically isolated from) the pipe212 if desired.

It is also within the scope of the present invention that any otherstrain sensing technique may be used to measure the variations in strainin the pipe, such as highly sensitive piezoelectric, electronic orelectric, strain gages attached to or embedded in the pipe 212.

While the sonar-based flow meter using an array of sensors to measurethe speed of sound of an acoustic wave propagating through the mixture,one will appreciate that any means for measuring the speed of sound ofthe acoustic wave may used to determine the entrained air volumefraction of the mixture/fluid.

It should be understood that, unless stated otherwise herein, any of thefeatures, characteristics, alternatives or modifications describedregarding a particular embodiment herein may also be applied, used, orincorporated with any other embodiment described herein.

Although the invention has been described and illustrated with respectto exemplary embodiments thereof, the foregoing and various otheradditions and omissions may be made therein without departing from thespirit and scope of the present invention.

1. An apparatus for determining a total gas content of a fluid flow passing through a pipe; said apparatus comprising: a bleed line coupled to the pipe for bleeding a portion of the fluid flow from the pipe at a bleed line pressure that is lower than the pressure of the fluid flow within the pipe, wherein the bleed line is sized to maintain flow rate sufficient to minimize slip between the gas and liquid phases of the fluid flow within a bleed test section such that a measured gas phase fraction of the fluid flow within the bleed test section is representative of the gas phase fraction of total gas content in the fluid flow in the pipe; a meter that determines the gas phase fraction of the fluid flow passing through the bleed test section of the bleed line.
 2. The apparatus of claim 1, wherein the total gas content includes entrained gas and dissolved gas in the pipe.
 3. The apparatus of claim 1 wherein the pressure in the bleed test section of the bleed line is at or near ambient pressure.
 4. The apparatus of claim 1, wherein the flow rate of the fluid flow through the bleed test section is several feet per second.
 5. The apparatus of claim 1, wherein the fluid flow passing through the bleed line flows continuously.
 6. The apparatus of claim 1, wherein the fluid flow passing through the bleed line flows periodically.
 7. The apparatus of claim 1, further includes a pump for returning the fluid flow in the bleed line back into the pipe.
 8. The apparatus of claim 22, wherein the fluid flow in the bleed line is not returned to the pipe.
 9. The apparatus of claim 1, wherein the flow rate of the fluid flow in the bleed line is sufficiently high to prevent stratification of the fluid flow in the bleed line.
 10. The apparatus of claim 1, further includes a bleed valve for directing a portion of the fluid flow to the bleed line.
 11. The apparatus of claim 1, further includes a second meter for measuring gas phase fraction of the fluid flow passing through the pipe; and a processor, responsive to both the gas phase fraction of the fluid flow in the pipe and the bleed line, that provides measurement of the dissolved gas content in the fluid flow passing in the pipe based on the difference between the gas phase fraction of the pipe and the gas phase fraction of the bleed line.
 12. The apparatus of claim 1, wherein the meter includes an array of sensors, having at least two sensors, for measuring the speed of sound propagating through the fluid flow in the bleed line to determine the gas phase fraction of the fluid flow in the bleed line.
 13. The apparatus of claim 12, wherein the array of sensors include a least 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 and 16 sensors.
 14. The apparatus of claim 13, wherein the sensors of the array of sensors are strain-based sensors.
 15. The apparatus of claim 12, wherein the meter determines the speed of sound passing through the fluid flow by determining the slope of an acoustic ridge in the k-ω plane.
 16. A method for determining a total gas content of a fluid flow passing through a pipe; said method comprising: bleeding a portion of the fluid flow from the pipe through a bleed line at a bleed line pressure that is lower than the pressure of the fluid flow within the pipe, wherein the bleed line is sized to maintain flow rate sufficient to minimize slip between the gas and liquid phases of the fluid flow within a bleed test section such that a measured gas phase fraction of the fluid flow within the bleed test section is representative of the gas phase fraction of total gas content in the fluid flow in the pipe; determining the gas phase fraction of the fluid flow passing through the bleed test section of the bleed line, wherein the gas phase fraction is representative of the gas phase fraction of total gas content in the fluid flow in the pipe.
 17. The method of claim 16, wherein the total gas content includes entrained gas and dissolved gas in the pipe.
 18. The method of claim 16, wherein the pressure in the bleed test section of the bleed line is at or near ambient pressure.
 19. The method of claim 16, wherein the flow rate of the fluid flow through the bleed test section is several feet per second.
 20. The method of claim 16, wherein the fluid flow passing through the bleed line flows continuously.
 21. The method of claim 16, wherein the fluid flow passing through the bleed line flows periodically.
 22. The method of claim 16, further includes returning the fluid flow in the bleed line back into the pipe using a pump.
 23. The method of claim 16, further includes not returning the fluid flow in the bleed line back to the pipe.
 24. The method of claim 16, wherein the flow rate of the fluid flow in the bleed line is sufficiently high to prevent stratification of the fluid flow in the bleed line.
 25. The method of claim 16, further includes directing a portion of the fluid flow to the bleed line using a bleed valve.
 26. The method of claim 16, further includes measuring gas phase fraction of the fluid flow passing through the pipe; and a processor, responsive to both the gas phase fraction of the fluid flow in the pipe and the bleed line, and determining the dissolved gas content in the fluid flow passing in the pipe based on the difference between the gas phase fraction of the pipe and the gas phase fraction of the bleed line.
 27. The method of claim 16, wherein determining the gas phase fraction includes determining the gas phase fraction using a meter that includes an array of sensors, having at least two sensors, for measuring the speed of sound propagating through the fluid flow in the bleed line to determine the gas phase fraction of the fluid flow in the bleed line.
 28. The method of claim 27, wherein the array of sensors include a least 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 and 16 sensors.
 29. The method of claim 28, wherein the sensors of the array of sensors are strain-based sensors.
 30. The method of claim 27, further including determining the speed of sound passing through the fluid flow by determining the slope of an acoustic ridge in the k-ω plane. 